Contents

Definition of "sense"

There is no firm agreement among
neurologists as to exactly how many senses there are, because of differing
definitions of a sense. In general, one can say that a
"sense" is a faculty by which outside stimuli are perceived. School
children are routinely taught that there are five senses (sight, hearing,
touch, smell, taste; a classification first devised by Aristotle),
though it is generally agreed that there are at least nine different senses in
humans, and a minimum of two more observed in other organisms.

A broadly acceptable definition of a sense
would be "a system that consists of a sensory cell type (or group of cell
types) that respond to a specific kind of physical energy, and that correspond
to a defined region (or group of regions) within the brain where the signals
are received and interpreted." Where disputes arise is with regard to the
exact classification of the various cell types and their mapping to regions of
the brain.

List of Human senses

Using this definition several senses can
be identified.

SIGHT
or vision describes the ability to detect electromagnetic energy within the
visible range (light)
by the eye and the brain
to interpret the image as "sight." There is disagreement as to
whether this constitutes one, two or even three distinct senses.
Neuroanatomists generally regard it as two senses, given that different
receptors are responsible for the perception of colour (the frequency of
photons of light) and brightness (amplitude/intensity - number of photons of
light). Some argue that the perception of depth also constitutes a sense, but
it is generally regarded that this is really a cognitive (that is,
post-sensory) function of brain to interpret sensory input to derive new
information.

HEARING
or audition is the sense of sound
perception and results from tiny hair fibres in the inner ear
detecting the motion of a membrane which vibrates in response to changes in
the pressure exerted by atmospheric particles within (at best) a range of 9 to
20000 Hz, however this changes for each individual. Sound can also be detected
as vibrations conducted through the body by tactition.
Lower and higher frequencies than can be heard are detected this way only.

TASTE
or gustation is one of the two main "chemical" senses. It is
well-known that there are at least four types of taste "bud"
(receptor) on the tongue
and hence, as should now be expected, there are anatomists who argue that
these in fact constitute four or more different senses, given that each
receptor conveys information to a slightly different region of the brain.

The four well-known receptors detect
sweet, salt, sour, and bitter, although the receptors for sweet and bitter
have not been conclusively identified. A fifth receptor, for a sensation
called umami,
was first theorised in 1908 and its existence confirmed in 2000 (see [1]).
The umami receptor detects the amino
acid glutamate, a flavor commonly found in meat, and in artificial
flavourings such as monosodium glutamate.

SMELL
or olfaction is the other "chemical" sense. Unlike taste, there are
hundreds of olfactory receptors, each binding to a particular molecular
feature, according to current theory. The combination of features of the odor
molecule makes up what we perceive as the molecule's smell. In the brain,
olfaction is processed by the olfactory
system. Olfactory
receptor neurons in the nose
differ from most other neurons in that they die and regenerate on a regular
basis.

TOUCH
or tactition is the sense of pressure perception, generally in the skin.
There are a variety of pressure receptors that respond to variations in
pressure (firm, brushing, sustained, etc).

THERMOCEPTION
is the sense of heat and the absence of heat (cold), also by the skin
and including internal skin passages. There is some disagreement about how
many senses this actually represents - the thermoceptors in the skin are quite
different from the homeostatic
thermoceptors which provide feedback on internal body temperature.

NOCICEPTION
is the perception of pain. It can be classified as from one to three senses,
depending on the classification method. The three types of pain receptors are
cutaneous (skin), somatic (joints and bones) and visceral (body organs). For a
considerable time, it was believed that pain was simply the overloading of
pressure receptors, but research in the first half of the 20th century
indicated that pain is a distinct phenomenon that intertwines with all other
senses, including touch.

Other

EQUILIBRIOCEPTION
is the perception of balance and is related to cavities containing fluid in
the inner ear. There is some disagreement as to whether this also includes the
sense of "direction" or orientation. However, as with depth
perception earlier, it is generally regarded that "direction" is a
post-sensory cognitive awareness.

PROPRIOCEPTION/KINESTHESIA
is the perception of body awareness and is a sense that people rely on
enormously, yet are frequently not aware of. More easily demonstrated than
explained, proprioception is the "unconscious" awareness of where
the various regions of the body are located at any one time. (This can be
demonstrated by anyone's closing the eyes and waving the hand around. Assuming
proper proprioceptive function, at no time will the person lose awareness of
where the hand actually is, even though it is not being detected by any of the
other senses).

Non-human senses

Other living organisms have receptors to
sense the world around them, including many of the senses listed above for
humans. However, the mechanisms and capabilities vary widely. Among non-human
animals, dogs have
a much keener sense of smell than humans, although the mechanism is similar. Pit
vipers and some boas
have organs that allow them to detect infrared
light, such that these snakes are able to sense the body heat of their prey.
The common
vampire bat may also have an infrared sensor on its nose (see here).
Infrared senses, are however, just sight in a different frequency range. Insects
have olfactory receptors on their antennae.
Ctenophores
have a balance receptor (a statocyst)
that works very differently from the mammalian semi-circular canals. In
addition, some animals have senses that humans do not, including the
following:

Magnetoception
(or "magnetoreception") is the ability to detect fluctuations in magnetic
fields and is most commonly observed in birds,
though it has also been observed in insects such as bees.
Although there is no dispute that this sense exists in many avians
(it is essential to the navigational abilities of migratory birds), it is not
a well-understood phenomenon (see [2]).
Recently, however, special implants have granted humans this ability too (see [3]).

Electroception
(or "electroreception"), the most significant of the non-human
senses, is the ability to detect electric
fields. Several species of fish, sharks
and rays have evolved the capacity to sense changes in electric fields in
their immediate vicinity. Some fish passively sense changing nearby electric
fields, some generate their own weak, electric fields and sense the pattern of
field potentials over their body surface, and some use these generating and
sensing capacities for social communication. The mechanisms by which
electroceptive fishes construct a spatial representation from very small
differences in field potentials involve comparisons of spike latencies from
different parts of the fish's body.

The only order of mammals that is known to
demonstrate electroception is the monotreme
order. Among these mammals, the platypus
(see [4])
has the most acute sense of electroception.

Humans (and probably other mammals) can
detect electric fields indirectly by detecting the effect they have on hairs.
An electrically charged balloon, for instance, will exert a force on human arm
hairs, which can be felt through tactition and identified as coming from a
static charge (and not from wind or the like). This is however not
Electroception since there is no separate sense for it. The presence of an
electrical field is merely concluded from a side-effect of another sense.

Magnetotactic
bacteria build miniature magnets inside themselves and use them to
determine their orientation relative to the Earth's magnetic field.

Echolocation
is the ability to determine orientation to other objects through
interpretation of reflected sound (like sonar).
Bats and cetaceans
are noted for this ability, though some other animals use it, as well. It is
most often used to navigate through poor lighting conditions or to identify
and track prey. There is presently an uncertainty whether this is simply an
extremely developed post-sensory interpretation of auditory perceptions or it
actually constitutes a separate sense. Resolution of the issue will require
brain scans of animals while they actually perform echolocation, a task that
has proven difficult in practice.

Pressure detection uses the lateral
line, which is a pressure-sensing system of hairs found in fish and some
aquatic amphibians.
It is used primarily for navigation, hunting, and schooling.

The Twelve Senses

In the 1920s, at a time when conventional
physiology only recognized five or six senses, Rudolf
Steiner proposed that there were far more. He defined twelve senses; of
these twelve, the first nine are now well-recognized: the equilibrioceptive
(balance), proprioceptive (movement), nociceptive (sense of pain and
wellness), tactile (touch), gustatory (taste), olfactory (smell),
thermoceptive (warmth), visual and auditory senses. Steiner proposed three
more senses as well: the sense of phoneme or language, the sense of thought
and the sense of ego (the ability to recognize an ego outside of our own); he
termed these three 'higher senses' that depended upon the healthy development
of the foundational senses of balance, movement, pain/wellness and touch.
Steiner's last three senses have not been confirmed by scientific research; in
particular, sensory organs for them would have to be found.

The Senses and Intelligence

Out of research into how creativity
manifests in different individuals, Howard
Gardner described multiple kinds of intelligence: visual, musical,
logical/mathematical, linguistic, movement, naturalistic, kinesthetic,
intrapersonal and interpersonal intelligences. Most of these correspond to
particular senses - in fact, all of them may if Steiner's higher senses
are included. Other senses (taste, smell) may also have their own particular
intelligences (Gardner has said that his search for additional intelligences
continues). The relationship between intelligence and sensory perception thus
appears to be a close one.

The relationship can become too close,
leading to a dominance of reactive over contemplative intelligence. Thus,
highly intelligent people are often described as 'quick' and 'sharp,'
suggesting the highly-tuned (even frazzled) nature of their nervous
system.